This invention relates generally to improvements in television receivers, and more particularly to a system for straightening the edges of a television image to eliminate the effects caused by pincushion distortion.
Present television receivers normally include circuitry which compensates for the so-called "pincushion" effect. Such compensation is conventionally achieved, at least to some extent, by proper design of the receiver's deflection yoke and/or by properly modulating the current in the deflection yoke. See, for example, U.S. Pat. Nos. 3,962,603, 4,225,809, 4,254,365 and 4,254,366, and U.K. Pat. No. 1,585,465. Although such pincushion correction schemes provide acceptable television images, they do not usually provide a completely corrected image. More nearly perfect image correction can be provided only with a substantial cost penalty.
The imperfections in the present state of pincushion correction are usually not detectable by a viewer who sees only the active trace area of the image. The border or edges of the image, where pincushion effects are quite noticeable, is usually masked by an escutcheon or the receiver cabinet.
In conventional projection television systems, the border of the projected image, and the pincushion distortion which occurs there, may be masked by causing the image edges to overlie a non-reflective border on the projection screen. Hence, the viewer is normally unaware of the edge distortion which would otherwise be very obvious in such a magnified image. As discussed below, however, the inclusion of a non-reflective border is sometimes impractical.
In certain large scale projection television systems where, for example, a television image is projected onto a wall to develop a six foot by eight foot image, it is impractical to provide a large non-reflective border on the wall to mask the imperfection in the edges of the image. If no provision for edge correction is provided, however, the distorted edges of the projected image will be readily apparent, and probably objectionable, to a viewer.
A related problem arises from the manner in which pincushion correction is typically achieved. In the usual case, vertical rate pulses are applied to circuitry which converts each such pulse to a parabolic waveform. The parabola, in turn, may be used to modulate the current in the receiver's deflection yoke. The circuitry which develops the parabola frequently requires active devices and/or multiple phase inputs, both of which are undesirable. In addition, high insertion loss and a lack of symmetry in the parabola can result from the use of such conventional circuits. Hence, edge correction of the type considered herein, which preferably employs parabola generating circuitry, is neither efficiently nor accurately achieved with conventional circuity.
For the foregoing reasons, conventional circuitry and systems for effecting pincushion correction are neither cost effective nor readily applicable to the problem of straightening the edges of a large projected television image.